BAGHDAD, July 9 -- Suspected Shiite militiamen have begun using powerful rocket-propelled bombs to attack U.S. military outposts in recent months, broadening the array of weapons used against American troops.

U.S. military officials call the devices Improvised Rocket Assisted Munitions, or IRAMs. They are propane tanks packed with hundreds of pounds of explosives and powered by 107mm rockets. They are often fired by remote control from the backs of trucks, sometimes in close succession. Rocket-propelled bombs have killed at least 21 people, including at least three U.S. soldiers, this year.

The latest reported rocket-propelled bomb attack occurred Tuesday at Joint Security Station Ur, a base in northeastern Baghdad shared by U.S. and Iraqi soldiers. One U.S. soldier and an interpreter were wounded in the attack.

U.S. military officials say IRAM attacks, unlike roadside bombings and conventional mortar or rocket attacks, have the potential to kill scores of soldiers at once. IRAMs are fired at close range, unlike most rockets, and create much larger explosions. Most such attacks have occurred in the capital, Baghdad.

The use of the rocket-propelled bombs reflects militiamen's ability to use commonly available materials and relatively low-tech weaponry to circumvent security measures that have cost the U.S. military billions of dollars. To combat roadside bombs, known as improvised explosive devices or IEDs, U.S. and Iraqi troops have set up scores of checkpoints throughout the capital, increased patrols and purchased hundreds of armored vehicles that can resist such attacks.

A June report on the Web site Long War Journal called the explosives-filled propane tanks "flying IEDs."

Militia members and insurgents have at times increased the sophistication of their weapons, but the rocket-propelled bombs are makeshift devices that also have been used in recent years by insurgents in Colombia. Propane tanks are ubiquitous in Iraq, where the fuel is widely used for cooking, making it hard for security forces to stop production of the bombs.

U.S. military officials in Baghdad have noted the use of rocket-propelled bombs in press releases in recent months. But they have not publicly discussed their use or their concerns about the weapons at length because most of the information about them is classified, U.S. military officials said.

"IRAM attacks could be very tragic against us," said Col. William B. Hickman, the commander of the 2nd Brigade Combat Team of the 101st Airborne Division, which operates in northwestern Baghdad. "We take them very seriously."

As the number of U.S. soldiers in Baghdad has begun to drop with the end of the "surge" of additional forces, U.S. military officials are placing a higher percentage of their troops in small outposts in densely populated neighborhoods. U.S. military officials say this is crucial to ensure the continued training of Iraq's security forces, win the trust of the capital's residents and improve local governance. But deployments in small outposts -- some are manned by just one platoon -- also have made soldiers more vulnerable.

To counter the threat posed by rocket-propelled bombs, soldiers have stepped up patrols around outposts, fortified their buildings and offered tens of thousands of dollars for information about networks that use the weapon.

The weapon first emerged as a threat here last fall and has become a top concern in recent months following a series of deadly attacks.

Most such attacks have been carried out during the day and some have been videotaped and aired on the satellite television station operated by Hezbollah, a Lebanese militia and political movement. U.S. military officials said they have found Iranian-made 107mm rockets at some of the blast sites, which they said suggests the weapons -- or parts -- may have come from Iran.

The deadliest reported rocket-propelled bomb attack occurred June 4 in the Shaab neighborhood of eastern Baghdad. U.S. soldiers stationed at a small base called Forward Operating Base Callahan heard a series of blasts shortly after 2 p.m.

The explosions were caused when a rocket on the back of a small flatbed truck exploded, igniting the other four to five IRAMs on the truck, the U.S. military said. The attack killed 18 Iraqis, wounded 29 and damaged 15 buildings, the military said.

"It is believed that the intended targets were U.S. soldiers at FOB Callahan and while in the final stages for the attack, for an unknown reason one rocket prematurely detonated causing the remaining rockets to explode erratically," the military said in a statement.

U.S. military officials said two suspected assailants were killed in the attack, describing them as members of "special groups" or Iranian-backed Shiite militias.

The second-deadliest attack occurred April 28 at Forward Operating Base Loyalty in eastern Baghdad. The attack, which took place shortly after 1 p.m., killed three soldiers. Militiamen fired 14 rocket-propelled bombs from the back of a cargo truck.

That same day, Joint Security Station Thawra, the U.S. military's only outpost in Sadr City, in eastern Baghdad, was also attacked with rocket-propelled bombs.

A man walked into an office at the station where Iraqis can file claims for compensation, and told soldiers of a U.S. military civil affairs unit that his truck had sustained damage during recent clashes between American troops and militiamen, according to two U.S. military officials who described the attack, speaking on the condition of anonymity because the information has not been publicly released.

The soldiers told the man to drive his truck close to the station so they could inspect it. The man pulled up in a small delivery truck containing eight IRAM launch tubes. The station was attacked soon afterward. At least 15 soldiers were wounded.

Although most rocket-propelled bomb attacks have taken place in eastern Baghdad, a joint security station in northern Hurriya, western Baghdad, was targeted last month.

On June 24, at approximately 3 p.m., a man walked up to the gate of the base and told soldiers, "You have a truck over there that goes boom," according to Capt. Jeremy Ussery, a company commander with the 101st Airborne Division's 1st Battalion, 502nd Infantry Regiment.

When soldiers inspected the truck, they realized an attempt to ignite nine IRAMs, with roughly 200 pounds of explosives apiece, had been unsuccessful.

"Just think of what could have happened," Ussery said. "Eighteen hundred pounds of explosives dispersed over this JSS."

The battalion approved a $50,000 reward for information on the attack and soldiers have handed out fliers to residents seeking tips. Ussery said the battalion typically offers rewards in the $10,000 range for "high value individuals," or prominent wanted militiamen.

The amount offered for information on the IRAM attack "is a very large reward," he said. "It's a small price to pay."